New study reveals real reason for Millennium Bridge sway

The pedestrian crossing over the Thames between Tate Modern and St Paul’s Cathedral opened in June 2000, with thousands of people lining up to walk it. Sadly, it was forced to shut down two days later as users experienced a disturbing influence in the structure as they passed through.

It remained closed for two years while engineers, Arup, investigated the unexpected oscillations. It was ultimately concluded that the phenomenon could not be stopped, and instead it was decided that the oscillation would be mitigated by the modernization of 37 viscous fluid dampers – 17 chevron dampers under the walkway to control the movement. lateral, 16 pillar shock absorbers to control lateral and torsional movement and four vertical shock absorbers to control lateral and vertical movements.

Experts at the time claimed that the movement of the bridge was caused by a phenomenon called synchronous lateral arousal – when people on the bridge subconsciously begin to walk in synchronization.

However, the new study, published in the scientific journal Nature Communication, says that is wrong.

In their report, experts from Bristol and Georgia explain that they have investigated many other bridges with similar oscillations and found little or no evidence of synchronicity between walkers.

Instead, they claim that the swing of the bridge is actually caused by pedestrians trying not to fall. Pedestrians randomly walking across the bridge provide “negative damping” whereby the energy of a person’s oscillation is transferred to the bridge.

This conclusion was obtained through observational and experimental evidence, rigorous new mathematical analysis, and detailed computer simulation.

Large oscillations can occur on a range of bridges. Now, armed with this knowledge, future engineers can avoid unwanted oscillations by ensuring that the frequencies of their bridges are not aligned with the typical frequency of footsteps.

John Macdonald, professor in the Department of Civil Engineering at the University of Bristol, said: “It was not the shape of the London Millennium Bridge that caused the problem. These large oscillations can occur on virtually any long bridge when transporting a large enough crowd.

“It turns out that the forces of many random steps left and right don’t cancel each other out, but the positive feedback causes the vibrations to get out of hand, much like when two or more laptops are too close to each other. the other on a Zoom call, which is ironic because most of this work has been done on Zoom with our collaborators in Cambridge, Atlanta and Wroklaw.

Alan Champneys, Professor in the Bristol Engineering Department, said: “This international multi-university collaboration has had a long history, but it shows the unique power of interdisciplinary collaboration between practicing engineers, mathematicians and physicists.

“Sometimes the answer is in plain sight, but the wisdom of the crowd has led for many years to an incorrect explanation of what is a very simple idea.”

Igor Belykh, professor at Georgia State University, said: “I have long been fascinated by the mathematical theory of synchronization and tried to apply the theory to bridge the instability, but this was n is that after interacting with colleagues at the University of Bristol that I realized there was a different story, which was extremely fun to finally understand together even though, due to the global pandemic, the Most of our work has been done on Zoom. “

Read the whole Emergence of unstable London Millennium Bridge without synchronization paper here.

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